1. Field of the Invention
The present invention relates generally to exhaust systems for motor vehicles and, more specifically, to an on-board reductant delivery system for an exhaust system of a motor vehicle.
2. Description of the Related Art
Exhaust systems for motor vehicles are known. Typically, the exhaust system includes an exhaust manifold connected to an engine of the motor vehicle and a close coupled catalyst connected via an exhaust pipe to the exhaust manifold. The exhaust system also includes an underbody catalyst connected via the exhaust pipe to the close coupled catalyst and a tail pipe connected to the underbody catalyst.
Lean NOx catalysts (LNC) and selective catalytic reduction catalysts (SCR) are known to reduce NOx emissions from diesel and lean-burn gasoline engines. In order to promote NOx reduction under oxidizing environments, certain reductants such as hydrocarbon (HC) fuels and/or urea solution or other ammonia (NH3) containing compounds must be added. To function effectively, the conditions for adding these reducing compounds to the exhaust system are critical. Specifically, the injected quantity must precisely match the NOx concentration in order to obtain maximum conversion and to avoid HC and/or NH3 slippage. Also, the response time must be fast to deal with the sudden increase of NOx during acceleration. Further, the fluid droplet size must be very fine and well distributed across catalyst surfaces to achieve high NOx conversion efficiencies. In addition, the system must be compact, light-weight, cost and energy efficient.
One known reductant delivery system uses a solenoid valve to meter diesel fuel, which was then dripped into the exhaust pipe. Another known reductant delivery system includes an injection system using a large fuel pump with a pressure regulator, a return line and an injector to meter and inject fuel into the exhaust. Yet another known reductant delivery system used a fuel pump, a fuel injector and an air assisted spray system. However, these systems are either slow, less effective in fluid dispersion, cumbersome, heavy, costly, noisy or high energy consumption.
Although the above reductant delivery systems have worked, it is desirable to improve NOx conversion efficiencies of selected catalysts under oxidizing conditions by delivering precise amount of reductants with fast response time and good distribution. It is also desirable to provide a low cost, robust reductant delivery system to control NOx emissions of vehicles equipped with lean NOx catalyst and/or selective catalytic reduction catalysts. It is further desirable to provide a compact, lightweight, energy efficient and potentially durable reductant delivery system for on-board vehicle application. Therefore, there is a need in the art to provide an on-board reductant delivery system for a motor vehicle.